1. Field of the Invention
The present invention relates to a durable, highly reflecting silver and aluminum multilayer used for mirrors reflecting light over a wide spectral range from infrared to ultraviolet.
2. Description of Related Art
Reflective metal coatings, particularly those made of silver, are susceptible to attack by oxygen and constituents of atmospheric pollution, such as chlorine, sulfur, and ozone. When these substances react with the reflective coating, the silver layer becomes tarnished so that the required optical properties of that layer are lost. Specifically, the tarnish lowers the effective reflectivity of the coating, rendering it unacceptable where unattended high reflectivity mirrors are needed. In addition, silver is mechanically very soft and hence easily abraded. These abrasions can destroy the reflective surface by increasing the dispersion of light or simply by removing the reflective silver coating.
It is known in the art to apply protective layers to these silver layers, where the nature of the protective layer is determined by cost and the required properties of the coated substrate. For example, transparent silver layers, such as may be used in solar control coatings, are protected against corrosion by overcoating them with one or more transparent metal oxide layers. Many other methods of corrosion resistance have been developed using layers of a variety of organic and inorganic substances.
U.S. Pat. No. 5,361,172 to Schissel et al. discloses protective outer layers of polymeric material and a metal oxide layer, and an adhesive layer between the substrate and silver layer.
U.S. Pat. No. 5,699,188 to Gilbert et al. discloses a multilayered polymer film with a reflective metal layer.
U.S. Pat. No. 4,009,947 to Nishida et al. discloses a protective layer made of an alloy of copper and tin.
U.S. Pat. Nos. 5,548,440 and 5,751,474 to Hohenegger et al. describe a protective coating made of a zinc sulfide layer and an intermediate barrier layer made of an oxide, fluoride, or oxinitride.
U.S. Pat. Nos. 5,583,704 and 5,424,876 to Fujii disclose protective layers of oxides and chromium sulfide for silver and aluminum mirrors.
U.S. Pat. No. 4,963,012 to Tracy et al. discloses a protective diffusion barrier for metal mirrors provided by a layer of silicon nitride.
U.S. Pat. Nos. 5,514,476 and 5,344,718 to Hartig et al. disclose a transparent glass coating system that includes a layer of silver between layers of nickel or nichrome, with an overcoat and an undercoat of silicon nitride.
U.S. Pat. No. 5,019,458 to Elgat et al. discloses multilayers with alumina or chromium-nickel alloy on a substrate, silver deposited thereon, with zinc sulfide and oxides on top of the silver layer.
Wolfe et al. have designed durable thin film coatings that permit light transmission in the visible range while reflecting infrared radiation. U.S. Pat. Nos. 5,377,045 and 5,521,765 to Wolfe et al. disclose thin film designs having a first layer of oxide, a layer of nickel chromium alloy, a silver layer, another layer of nickel chromium alloy, and a top layer of silicon nitride. U.S. Pat. No. 5,563,734 to Wolfe discloses silver layers sandwiched between layers of nickel-chromium nitride and silicon nitride with a further oxide outer layer. These thin films are used as filters, or substrates that are transparent to visible light, but block out infrared radiation. These filters are used in display devices or transparent panels in buildings, vehicles, or other structures to prevent some, but not all, solar radiation from penetrating the substrate. Since the purpose of the filters is to transmit visible light, they are inadequate when the reflection of visible light, as well as infrared and ultraviolet light, is needed.
U.S. Pat. No. 5,215,832 to Hughes et al. discloses plate glass mirrors that reflect visible light and contain reflective metal layers and a zinc-containing polymeric protective layer (paint) on the back of a transparent glass substrate. Incident light passes through the glass substrate and is reflected back by the reflective metal layers underneath. Hughes et al. describe how a variety of metals or metallic compounds (chromium, copper, stainless steel, titanium nitride) might be used in combination to decrease the transmission of the final film. However, some of these materials have optical constants that make them absorbing, which reduces the reflectance of the mirror. In addition, they will not confer environmental durability to the mirror. Hughes et al. do not discuss the design of mirrors having high reflectance over a broadband of wavelengths, which is addressed by the present invention.
A need exists for reflective surfaces, or mirrors, that reflect light over a wide spectral region and are resistant to corrosion from contaminants or humidity. The present invention addresses the need for such mirrors, particularly in optical systems requiring high reflectance mirrors to maximize optical throughput. A thin film multilayer is provided using silver and aluminum layers in combination to reflect visible, ultraviolet, and infrared light.